In order to store information it is known to use a magnetic tape storage system in which a magnetic tape is wound on reels or spools. These magnetic tape information storage systems are extensively used to back up archive and store data for future use by a user of, for example, a host user interface or PC.
A typical magnetic tape storage device 100 is illustrated in FIG. 1. The tape storage device 100 may be a stand alone unit, or may be integrated within a casing of a host computer entity 101. The data storage device is operable to receive data from a host computer entity and store data on a magnetic tape data storage medium, contained within a tape cartridge, and also to read data from cartridges, and input read data to the host computer 101.
Referring to FIG. 2 herein, there is shown schematically in external view, a typical known tape data storage cartridge 200, comprising a casing 201 containing in this case, a single reel upon which is wound a length of magnetic tape data storage medium. The magnetic tape data storage medium is wound in and out of the cartridge through an aperture 202, onto a reel within the tape data storage device in use.
Referring to FIG. 3 herein, there is illustrated schematically a cartridge 200 inserted into a tape data storage device 100, wherein a length of tape data storage medium 300 is wound from an internal reel 301 of the cartridge, through a series of capstans and rollers 302-305 onto a second reel 306 comprising the tape data storage device. The tape data storage device comprises a read/write head 307 over which the tape is drawn, in forward and reverse directions, to apply read or write operations of user data to the tape.
Typically, the tape drive controls the movement of the tape over the write head to record data onto the magnetic tape, and over the read head to generate an electrical signal from which the stored data can be reconstructed. Commonly, the read and write heads may be combined into a single read/write head, this head being controlled by the tape drive.
A length of magnetic tape as known in the art is illustrated in FIG. 4 such that data is recorded onto the tape in a series of data tracks 400. When a command is issued by a host computer to read a specific target data on the tape the tape drive using a read head must scan the data tracks 400 to locate the position of the target data thereby allowing the read head to retrieve the data and transfer it back to the host. The tape drive, being configured to ascertain a current position on the tape relative to a Beginning Of Tape or Wrap (BOW), scans the data tracks 400 until the read head passes over a directory 401 positioned at the BOW.
In the example shown here, the directory being located at the BOW is configured such that its contents are distributed across the tape, at the end of each Wrap as shown in FIG. 4. The contents of the directory 401 coincide to the separate data tracks 200, such that the contents of the directory 401 located at the BOW or the EOW are used to allow the tape drive to determine if the target data is contained on a relevant data track. If data position information in the area 401 indicates that the target data is not located in a particular data track then the drive must continue reading the data until it comes across the target data. Similar prior art data storage systems utilize a directory stored in a cartridge memory rather than on the tape, data being accessed on tape by a read operation of the cartridge memory.
When recording data onto a magnetic tape it is known to partition the data into a plurality of data sets, such data sets being distributed across the various data tracks. The partitioning of data into data sets distributed across the data tracks provides a physical position of any one particular data set relative to, for example the BOW and EOW. Such a physical positioning being provided as the data sets are spatially separated along the length of the tape.
Within one particular data set the data is further partitioned into a series of records and filemarks, such partitioning giving rise to a logical data position for any particular record or filemark.
Magnetic tape data storage systems known in the art having directories 401 associated with a corresponding data track 400 along the length of a tape, utilize such directories to store logical data position information as detailed in FIG. 5. A data track 400 is illustrated as having data 500 distributed across its length. The data 500 within a data set and positioned on a data track 400 has corresponding data position information stored within the directory as record data position information 501 and filemark data position information 502. Commonly, the directories also contain data position information 503 relating to the positioning of data sets distributed across the length of the data track 400.
Referring to FIG. 6 there is detailed a typical mode of operation of a prior art magnetic tape data storage system having received a target data command from a host. Using the directory located at the BOW, and in particular the contents of the directory located at the BOW and EOW corresponding to a particular data track, the tape drive determines a required tape motion so as to position the read head on a data track corresponding to the data track on which the target data is positioned. The directory contents information at the BOW and the EOW provide information on the positioning of the target data within this particular data track. As the read head is effectively transported to a new data track as shown in FIG. 6 the tape motion may be such that the read head is being transported away from the target data position B. The read head must serially read the data track until it is determined, using the contents of the directory information at the EOW (as shown in FIG. 6) that the tape motion should be reversed in order to allow access to the target data B. Essentially, each data track information area 401 functions as a map of the logical position of the data to the actual physical position of the data on the tape, this data being partitioned into data sets distributed along data tracks. Obviously, the more information a directory, located at the BOW contains, the more complete the mapping of the physical position of data on tape.
However, a prior art storage device having a complete directory (containing information relating to all data on tape) is restricted by the read speed of the tape drive due to the serial reading operation as detailed in FIG. 6 when the read head traverses from, for example, Al to target data B. Moreover, a directional change of tape motion is undoubtedly required at some stage of the data retrieval process, this adding to the data access time.
Accordingly, the inventors have identified various problems associated with the employment and utilization of such magnetic tape data storage systems as described above and known in the art. The problems identified by the inventors include:                Utilizing data position information in a distributed manner (the spatially separated directories 401) results in excess tape motion and in particular multidirectional changes. This excessive tape motion inevitably leads to a longer time period taken from the issuing of a target data request and the actual reading and retrieval of the target data.        Due to the operational nature of the magnetic tape systems, data on tape is often deleted or updated, if in fact the storage system is capable of data update operations. In the event of data being updated on the magnetic tape the data position information increasingly becomes out of data and hence the search and data access times are increased due to the system having a reduced awareness of a target data position.        The operational nature of the prior art magnetic tape data storage systems, involving multiple data tracks and corresponding multiple directories necessitates additional tape formatting so as to provide such areas for data position information storage. This additional formatting increases the time take for initialization of the magnetic tape to provide a fully functional magnetic tape data storage facility.The inventors have recognized a need for a magnetic tape data storage system having an updateable centralized data position information storage utility capable of storing data position information relating to data distributed across the tape. The method and apparatus of such a system having an accelerated target data searching utility in relation to that found in the art, is disclosed in detail herein below.        